[Django] #31614: Add a NULLS FIRST / LAST clause while ordering a union

[Django]

#31614: Add a NULLS FIRST / LAST clause while ordering a union
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               Reporter:  Laurent Tramoy  |          Owner:  nobody
                   Type:  Uncategorized   |         Status:  new
              Component:  Uncategorized   |        Version:  master
               Severity:  Normal          |       Keywords:
           Triage Stage:  Unreviewed      |      Has patch:  0
    Needs documentation:  0               |    Needs tests:  0
Patch needs improvement:  0               |  Easy pickings:  0
                  UI/UX:  0               |
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 I have read the recent tickets about unions and order_by
 (https://code.djangoproject.com/ticket/31496,
 https://code.djangoproject.com/ticket/27995,
 https://code.djangoproject.com/ticket/30628) , and my bug is slightly
 different, so I hope it's not a duplicate.

 Let's consider two similar models:

 {{{
 class EntityA(models.Model):
     name_a = models.CharField(max_length=128, null=True)
     dt_a = models.DateTimeField(null=True)


 class EntityB(models.Model):
     name_b = models.CharField(max_length=128, null=True)
     dt_b = models.DateTimeField(null=True)

 EntityA.objects.create(name_a="a")
 EntityA.objects.create(name_a="qwerty", dt_a=timezone.now())
 EntityB.objects.create(name_b="random", dt_b=timezone.now())
 EntityB.objects.create(name_b="b")

 qs_a = EntityA.objects.values(name=F("name_a"), dt=F("dt_a"))
 qs_b = EntityB.objects.values(name=F("name_b"), dt=F("dt_b"))

 # union queryset
 queryset = qs_a.union(qs_b)
 }}}
 I can use a simple ORDER BY clause:

 {{{
 queryset.order_by("-dt")
 }}}
 And everything will work, no problem here.

 What I actually want is the same query, but with a NULLS LAST
 Usually the query becomes:


 {{{
 queryset.order_by(F("dt").desc(nulls_last=True))
 }}}

 but that raises a
 {{{
 DatabaseError: ORDER BY term does not match any column in the result set.
 }}}

 I know unions can handle only a few clauses,  but ORDER BY is one of them,
 so I'm unsure whether this is the expected behaviour or not.
 If it's expected, then the raised exception could be more explicit.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/31614>
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